Authors: Respectively, College Associate Professor, Associate Professor, Research Technician, Research Specialist, College Associate Professor, all with the Agricultural Science Center at Farmington, New Mexico.

Plant growth and crop production are intrinsically linked to climate and daily weather conditions. Climate determines the types of plants that can be grown in a particular area and the management techniques required for successful landscapes and agricultural sustainability. Weather affects crop water requirements, timing of planting, cultivation, fertilization and harvest, crop tolerance to disease and pests, total crop yield and product quality. Agricultural and landscape planning begins with a thorough understanding of a locale's climate. To help provide this understanding for the Four Corners region, daily weather observations have been made since 1969 at the New Mexico State University (NMSU) Agricultural Science Center (ASC) at Farmington. The ASC is located in northwestern New Mexico (36° 4' N Lat., 108° 2' W Long.) about seven miles southwest of Farmington at an elevation of 5,640 feet above mean sea level. Two weather-data recording stations are located at the ASC. Station 1 (WS-1) was established at the center in January 1969 and was given an official designation by the National Weather Service (NWS) in 1978. Station 2 (WS-2) was installed approximately 400 yards south of WS-1 in 1985. This is an automated, computer-linked station that is operated and maintained by the New Mexico Climate Center (NMCC) at NMSU in Las Cruces. This report summarizes the weather observations from both of these stations between 1969 and 2003.

Summary

Compared to the mountainous areas that surround it, climate at the ASC is relatively dry and mild. Summer days are typically warm (90-95°F) and dry, while nights are cool (55-60°F). During the winter months of December and January, air temperatures commonly fall below 20°F in early morning, while daytime highs typically range between 35 and 45°F. The frost-free period averages 162 days from early May to mid-October, but crops such as potatoes, corn, spring cereals, etc. are usually planted before May 1. Throughout the year, days are typically clear and sunny. When precipitation events occur, they are usually of short duration and deposit less than 0.10 inch of rain per event. During the winter, snows are infrequent and snow depths greater than 6 inches are uncommon. Total annual precipitation averages 8.2 inches with about half of the total occurring in the so-called monsoon season (July-October). Twenty-four-hour rainfall amounts exceeding 1 inch have occurred less than 20 times during the entire 35 years of record. Some of the more extreme rainfall events were 1.5 inches on Oct. 22, 1969; 1.9 inches on Sept. 5-6, 1970; 2.7 inches on July 20-23, 1986; 1.9 inches on April 4-5, 1997; and 3.0 inches on Sept. 2-4, 2002. Compared to southern and eastern New Mexico, winds are relatively calm during the summer and winter, averaging about 110 miles per day (4.6 mph) between July and January. During March, April, and May, strong westerly winds are common and average wind velocity increases to about 150 miles per day (6.3 mph).

Materials and Methods

Weather Station 1

Air temperature measurements were recorded from standard U.S. Weather Bureau maximum and minimum thermometers housed in a regulation, louvered instrument shelter. A standard 8-inch diameter rain gauge was installed in 1969 to measure total daily precipitation. A battery-operated, constant-reading rain gauge was installed in 1982 to measure precipitation rate. Wind movement in miles per day has been recorded at two heights since 1980 using three-cup anemometers. One is located 6 inches above a standard Class-A metal evaporation pan, while the other is set at a height of 6 feet above the soil surface.

Water evaporation (pan) was measured daily from a Class-A pan with a hook-gauge from May 1 through September 30 in all years from 1974 to 2003. Pan measurements were also made throughout April in 1977 through 1981, 1986, and from 1992 through 2003. Complete October measurements were recorded in 1972, 1974 through 1978, 1984 and 1985, 1987 and 1988, 1990, and from 1992 through 2003.

Maximum and minimum bare-soil temperatures at a depth of 4 inches have been recorded since 1976, using buried temperature sensors. The soil type is a Doak fine sandy loam with about 70% sand, 20% silt, and 10% clay.

Using a pyranometer, daily solar radiation measurements were recorded from 1977 through 2003. Between January 1977 and September 1996, the instrument was set near the instrument shelter at a height of 6.5 feet. Subsequent readings were obtained at a 10-foot height from WS-2. Temperature and precipitation data recorded at WS-1 are summarized on the NWS Web site linked to: http://weather.nmsu.edu/nmcccooperator/farmington_ag_sci.htm.

Weather Station 2

The NMCC weather station consisted of air temperature, relative humidity, solar radiation, wind speed, wind direction, and rain-depth sensors wired to a Campbell CR10 datalogger. The datalogger, powered by a 12-volt, deep-cycle, lead-acid battery, and all sensors were mounted to a 3-meter high, steel and aluminum tripod. A solar-panel provided recharge to the battery. Measurements were recorded to the datalogger every 15 minutes, and the data were downloaded to a PC at the NMCC via modem and telephone line. WS-2 data are accessible from the NMCC Web site: http://weather.nmsu.edu/cgi-shl/cns/oldformat.pl.

Unless noted otherwise, data presented in this report are from WS-1.

Results

Air Temperature

Between 1969 and 2003, the daily minimum, mean, and maximum air temperatures averaged over all months and years were 38.7, 52.6, and 66.4°F, respectively (fig. 1, tables 1, 2, and 3). Average annual minimums ranged from 36.1°F in 1975 to 40.8°F in 2001 (table 1), while average maximums ranged from 63.7°F in 1982 to 69.3°F in 2003 (table 3). The highest annual mean temperature of 55.1°F during the 35-year period occurred in 2003 (table 2). This was primarily due to record or near record high average minimum (64, 62, and 44°F) and maximum (97, 91, and 74°F) temperatures for July, August, and October, respectively (tables 1 and 3). The coolest year on record (1975) appeared to be a result of lower than average minimum temperatures in May and June (table 1).

Figure 1. Average daily minimum, mean, and maximum air temperature for each year at NMSU's
Agricultural Science Center at Farmington, 1969-2003.

* Lowest average daily maximum temperature for month or year during 35-year period.
** Highest average daily maximum temperature for month or year during 35-year period.

January and December were the coldest months, having average daily minimum, mean, and maximum temperatures of about 20, 31, and 42°F, respectively (fig. 2). July and August had the highest average daily temperatures (60°F minimum, 75°F mean, and 90°F maximum).

Figure 2. Average daily minimum, mean, and maximum air temperature for each month at NMSU's
Agricultural Science Center at Farmington, 1969-2003.

Frost-Free Period

The frost-free period (consecutive days above 32°F) averaged 162 days over the 35 years (table 4). The shortest frost-free period occurred in 1999 (115 days), the only year in which a June frost was recorded. The longest frost-free period occurred in 1977 (193 days). The average dates of the last spring frost and first autumn frost were May 5 and Oct. 13, respectively. The earliest and latest dates of the last spring frost were April 10, 1990 and June 5, 1999, respectively. The earliest and latest dates of the first fall frost were Sept. 18, 1971 and Nov. 12, 1988 (table 4). The number of consecutive days without a killing frost (28°F or less) averaged 182. A killing frost results in substantial damage to most plants. A temperature of 28°F for more than 30 minutes, for example, will kill approximately 10% of apple or cherry blossoms during flowering (Longstroth, 2001).

Table 4. Frost dates and number of consecutive frost-free days at NMSU's Agricultural Science Center at Farmington, 1969-2003.

Extreme Temperatures

Although the average number of days between the first autumn frost and last spring frost was 204, the temperature dropped below 32°F on only 139 of those days (table 5). In the months of November through March, the temperature dropped below freezing about 25 days per month on average (table 5). The temperature dropped to zero or below 1.8 days per year on average between 1969 and 1992 but fell below zero only once since 1992 (table 5). The coldest temperatures recorded during the 35-year period were -18°F, -16°F, and -14°F and occurred in January 1971, December 1990, and February 1989, respectively (table 6).

Table 5. Number of days 32°F or below and 0°F or below in the winter, spring, and fall months at NMSU's Agricultural Science Center at Farmington, 1969-2003.

Year

------------------------ Number of Days 32°F or Below -------------------

The temperature exceeded 95°F about 13 days per year on average and, while it exceeded 100°F on only 28 days of the entire 35-year period, nine of the 28 days occurred in July 2003. The highest recorded temperature of 103°F occurred in July 1989, 1990, and 2003 (table 7).

Precipitation

From 1969 through 2003, the average annual precipitation was 8.16 inches (fig. 3 and table 8). Annual precipitation extremes ranged from a low of 3.57 inches in 1976 to a high of 14.65 inches in 1986 (fig. 3). The average monthly precipitation ranged from a low of 0.26 inches in June to 1.10 inches in August (fig. 4). The greatest amount of rainfall in a single month was 4.10 inches in July 1986. In 22 months over the 35 years, there was no measurable precipitation (table 8).

Wind

Since 1980, the average daily wind run at the 6-foot height has been 123.4 miles/day (5.1 mph). But it varied considerably between the spring months and rest of the year (fig. 5 and table 9). March and April were the windiest months, with an average daily wind run at the 6-foot height of more than 150 miles/day. The calmest months of August through December had an average daily wind run of about 110 miles (fig. 5). In 1991, March and April were particularly windy with an average daily wind run at the 6-foot height of 190 miles (table 9).

Figure 5. Average daily wind run at a height of 6 feet for each month at NMSU's Agricultural Science
Center at Farmington, 1969-2003.

Table 9. Average daily wind run (miles/day) measured at two heights (2 feet and 6 feet) during each month at NMSU's Agricultural
Science Center at Farmington, 1980-2003.

Solar Radiation

From 1977 to 2003, average daily solar radiation, measured as gram calories/cm2 (Langley's), ranged from a low of about 200/day in January and December to a high of about 625/day in June and July (fig. 6 and table 10). Total annual radiation averaged about 154,000 Langley's (421 Langley's/day).

Pan Evaporation

Evaporation is affected by solar radiation, temperature, humidity and wind. With appropriate correction factors, it is sometimes used as an index of plant water-use. From 1974 to 2003, total pan evaporation between May 1 and Sept. 30 averaged 57 inches (table 11) or 0.37 inches/day (table 12). Average total monthly evaporation was greatest in June and July (13.1 inches) (table 11 and fig. 7). This corresponded to an average daily evaporation rate of about 0.43 inches (table 12).

Table 12. Average daily evaporation (inches) in each month from a Class-A pan at NMSU's Agricultural Science Center at Farmington,
1972-2003.

Year

Apr.

May

June

July

Aug.

Sept.

Oct.

Mean*

1972

---

---

---

0.478

0.381

0.319

0.142

---

1973

---

---

0.370

0.372

0.344

0.319

---

---

1974

---

0.419

0.512

0.415

0.395

0.311

0.148

0.410

1975

---

0.299

0.401

0.396

0.403

0.270

0.242

0.354

1976

---

0.380

0.515

0.444

0.423

0.302

0.190

0.413

1977

0.304

0.396

0.498

0.423

0.394

0.317

0.213

0.406

1978

0.310

0.311

0.427

0.469

0.422

0.321

0.257

0.390

1979

0.278

0.278

0.362

0.354

0.342

0.317

---

0.331

1980

0.258

0.322

0.489

0.452

0.406

0.272

---

0.388

1981

0.254

0.297

0.470

0.388

0.363

0.255

---

0.355

1982

---

0.323

0.427

0.392

0.314

0.243

---

0.340

1983

---

0.323

0.384

0.404

0.357

0.291

---

0.351

1984

---

0.392

0.389

0.379

0.336

0.261

0.106

0.351

1985

---

0.282

0.409

0.409

0.375

0.233

0.143

0.341

1986

0.245

0.317

0.366

0.366

0.366

0.225

---

0.328

1987

---

0.214

0.382

0.393

0.335

0.274

0.101

0.320

1988

---

0.373

0.369

0.421

0.314

0.285

0.199

0.352

1989

---

0.393

0.418

0.446

0.356

0.312

---

0.385

1990

0.293

0.373

0.516

0.411

0.366

0.294

0.186

0.392

1991

0.289

0.377

0.366

0.412

0.358

0.284

---

0.360

1992

0.259

0.280

0.405

0.384

0.348

0.273

0.211

0.338

1993

0.322

0.339

0.465

0.477

0.328

0.304

0.180

0.383

1994

0.278

0.384

0.501

0.504

0.402

0.309

0.238

0.420

1995

0.249

0.315

0.424

0.445

0.375

0.325

0.241

0.377

1996

0.303

0.435

0.424

0.451

0.358

0.236

0.183

0.381

1997

0.246

0.301

0.395

0.399

0.309

0.259

0.187

0.333

1998

0.242

0.367

0.471

0.420

0.366

0.334

0.189

0.392

1999

0.277

0.347

0.437

0.379

0.280

0.274

0.240

0.343

2000

0.321

0.426

0.470

0.425

0.366

0.295

0.157

0.397

2001

0.282

0.366

0.464

0.405

0.353

0.362

0.235

0.388

2002

0.307

0.429

0.493

0.455

0.396

0.261

0.149

0.407

2003

0.274

0.374

0.493

0.504

0.397

0.311

0.212

0.416

Mean

0.280

0.348

0.436

0.420

0.363

0.289

0.189

0.371

*Mean daily evaporation from May through September.

Soil Temperature

The average daily bare soil temperature 4 inches below the surface did not exceed 50°F (the minimum seed germination temperature for many crops) until April of each year (fig. 8). In July and August, the daily mean temperature at this depth equaled or exceeded 80°F (fig. 8 and table 13). Average soil temperature extremes ranged from a low of 25.6°F in January (table 14) to a high of 101.0°F in July (table 15).

Growing Degree-Days

Growing degree-days (GDD) or heat units are used to rate the maturity of various plant cultivars, especially corn. Plant development is related to temperature, and each crop has an optimum temperature range for growth. That is, there is a minimum temperature below which plants will not grow (minimum cutoff or base temperature) and a maximum temperature at which plant growth rate will stabilize or decrease (maximum cutoff temperature). The average daily temperature (mean of daily minimum and maximum temperatures) minus the base temperature is equal to GDD for that day. However, if the observed minimum temperature for the day is less than the minimum cutoff temperature (COmin), then it is set equal to COmin. Correspondingly, if the observed maximum temperature exceeds the maximum cutoff temperature (COmax), then it is set equal to COmax prior to calculating the average. In corn, base and COmin is 50°F and COmax is 86°F (Eckert, 2004). Over the 35 years, total corn GDD from May 1 to Sept. 30 averaged 2,865 (table 16). Total average monthly GDD during this period ranged from a low of 400 in May (12.9/day) to a high of 711 (22.9/day) in July (fig. 9 and table 16).

Reference Evapotranspiration

Evapotranspiration (ET) refers to the volume of water used by a crop during a given time period. It includes water that is actively transported through the plant from the roots to leaves during transpiration and water that is lost from plant and soil surfaces through evaporation. If crops are healthy and soil moisture is not limiting, plant size and weather (radiation, air temperature, humidity, and wind speed) are the primary factors affecting crop ET (Allen, et al., 1998). Reference ET (ETo) refers to the ET of a reference crop such as grass or alfalfa that is of a certain height and is growing under optimum conditions for maximum production. Since ETo has been correlated with the weather parameters referred to above, it can be calculated when these parameters are available. Correction values or crop coefficients (Kc), if available, can then be applied to ETo to estimate a particular crop's ET rates or water requirements during the season. These estimates can then be used in irrigation scheduling. Since different researchers have derived unique mathematical formulas to calculate ETo, to effectively use published Kc values for irrigation scheduling, the ETo formula used to formulate the Kc must be known.

From 1996 to 2003, average daily ETo (using WS-2 data and a modified, grass-referenced Penman formula from the New Mexico Climate Center: http://weather.nmsu.edu/pmcomp.htm) ranged from 0.08 inch/day in January and December to 0.38 inch/day in June (fig. 10). Total annual ETo averaged 80.5 inches (table 17). From May through August, the active growing season for many crops, ETo averaged 10.4 inches/month or 0.34 inch/day.

Figure 10. Average daily reference evapotranspiration (ETo) at NMSU's Agricultural Science Center at
Farmington based on calculations from the New Mexico Climate Center, 1996-2003.

Table 17. Average total and average daily reference evapotranspiration* (ETo) during each month at NMSU's Agricultural Science
Center at Farmington, 1996-2003.

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